"Mysterious Girlfriend X" centers on the peculiar relationship between Tsubaki Akira, a high school student, and Mikoto Urabe, a mysterious transfer student. Urabe is known for her aloof demeanor, unusual habits, and a perpetually blank expression. The story begins when Tsubaki, out of curiosity, tastes a drop of drool left on a desk by Urabe after she falls asleep in class. This act triggers a strange connection between the two, as Tsubaki becomes inexplicably drawn to Urabe. Urabe reveals that her drool has a unique property, capable of creating a bond between individuals, and she begins to share it with Tsubaki regularly.

As their relationship develops, Tsubaki and Urabe navigate the complexities of their bond, which is both intimate and enigmatic. Urabe’s drool becomes a recurring motif, symbolizing their growing connection and the emotional intimacy they share. Despite their closeness, Urabe remains emotionally distant and cryptic, often leaving Tsubaki confused about her true feelings. The series explores their interactions in a slice-of-life format, blending moments of humor, romance, and surrealism.

The narrative delves into the dynamics of their relationship, with Tsubaki gradually becoming more attached to Urabe, while she maintains her enigmatic persona. Their bond is tested by various situations, including misunderstandings, jealousy, and the interference of other characters, such as Tsubaki’s childhood friend Oka Ayuko, who harbors feelings for him. Urabe’s past and the origins of her peculiar abilities are hinted at but remain largely unexplained, adding to the series’ air of mystery.

Throughout the story, the focus remains on the evolving relationship between Tsubaki and Urabe, with their interactions often oscillating between tender and bizarre. The series balances its surreal elements with grounded emotional moments, creating a unique blend of romance and psychological intrigue. The characters’ growth is subtle, with Tsubaki becoming more introspective and Urabe gradually revealing glimpses of vulnerability beneath her stoic exterior.

The manga concludes with an open-ended resolution, leaving the future of Tsubaki and Urabe’s relationship ambiguous. The story emphasizes the themes of connection, intimacy, and the complexities of human emotions, all while maintaining its distinctive tone and narrative style.